Apolipoprotein E—a protein that carries cholesterol in blood and that appears to play some role in brain function. The gene that produces this protein comes in several forms, or alleles: ε2, ε3, and ε4. The APOE ε2 allele is relatively rare and may provide some protection against AD (but it may increase risk of early heart disease). APOE ε3 is the most common allele and appears to play a neutral role in AD. APOE ε4 occurs in about 40 percent of all people with AD who develop the disease in later life; it increases the risk of developing AD.

Axon—the long extension from a neuron that transmits outgoing signals to other cells.

Beta-amyloid—a part of the amyloid precursor protein found in plaques, the insoluble deposits outside neurons.

Brain stem—the portion of the brain that connects to the spinal cord and controls automatic body functions, such as breathing, heart rate, and blood pressure.

Capillary—a tiny blood vessel. The brain has billions of capillaries that carry oxygen, glucose (the brain’s principal source of energy), nutrients, and hormones to brain cells so they can do their work. Capillaries also carry away carbon dioxide and cell waste products.

Cerebellum—the part of the brain responsible for maintaining the body’s balance and coordination.

Cerebral hemispheres—the largest portion of the brain, composed of billions of nerve cells in two structures connected by the corpus callosum. The cerebral hemispheres control conscious thought, language, decision making, emotions, movement, and sensory functions.

Cerebrospinal fluid—the fluid found in and around the brain and spinal cord. It protects these organs by acting like a liquid cushion and by providing nutrients.

Chromosome—a threadlike structure in the nucleus of a cell that contains DNA. DNA sequences make up genes. Most human cells have 23 pairs of chromosomes containing approximately 30,000 genes.

Clinical trial—a research study involving humans that rigorously tests safety, side effects, and how well a medication or behavioral treatment works.

Cognitive functions—all aspects of conscious thought and mental activity, including learning, perceiving, making decisions, and remembering.

Computed tomography (CT) scan—a diagnostic procedure that uses special x-ray equipment and computers to create cross-sectional pictures of the body.

Corpus callosum—thick bundles of nerve cell fibers that connect the two cerebral hemispheres.

Dementia—a broad term referring to a decline in cognitive function to the extent that it interferes with daily life and activities.

Dendrite—a branch-like extension of a neuron that receives messages from other neurons.

DNA (deoxyribonucleic acid)—a long, double-stranded molecule within the nucleus of the cell that forms chromosomes and genes.

Early-onset Alzheimer’s disease—a rare form of AD that usually affects people between ages 30 and 60. It is called familial AD (FAD) if it runs in the family.

Entorhinal cortex—an area deep within the brain where damage from AD often begins.

Enzyme—a protein that causes or speeds up a biochemical reaction.

Free radical—a highly reactive molecule (typically oxygen or nitrogen) that combines easily with other molecules because it contains an unpaired electron. The combination with other molecules sometimes damages cells.

Gene—the biologic unit of heredity passed from parent to child. Genes are segments of DNA and contain instructions that tell a cell how to make specific proteins.

Genetic risk factor—a variant in a cell’s DNA that does not cause a disease by itself but may increase the chance that a person will develop a disease.

Hippocampus—a structure in the brain that plays a major role in learning and memory and is involved in converting short-term to long-term memory.

Hypothalamus—a structure in the brain under the thalamus that monitors activities such as body temperature and food intake.

Late-onset Alzheimer’s disease—the most common form of AD. It occurs in people aged 60 and older.

Limbic system—a brain region that links the brain stem with the higher reasoning elements of the cerebral cortex. It controls emotions, instinctive behavior, and the sense of smell.

Magnetic resonance imaging (MRI)—a diagnostic and research technique that uses magnetic fields to generate a computer image of internal structures in the body. MRIs are very clear and are particularly good for imaging the brain and soft tissues.

Metabolism—all of the chemical processes that take place inside the body. In some metabolic reactions, complex molecules are broken down to release energy. In others, the cells use energy to make complex compounds out of simpler ones (like making proteins from amino acids).

Microtubule—an internal support structure for a neuron that guides nutrients and molecules from the body of the cell to the end of the axon.

Mild cognitive impairment (MCI)—a condition in which a person has memory problems greater than those expected for his or her age, but not the personality or cognitive problems that characterize AD.

Mutation—a permanent change in a cell’s DNA that can cause a disease.

Myelin—a whitish, fatty layer surrounding an axon that helps the axon rapidly transmit electrical messages from the cell body to the synapse.

Nerve growth factor (NGF)—a substance that maintains the health of nerve cells. NGF also promotes the growth of axons and dendrites, the parts of the nerve cell that are essential to its ability to communicate with other nerve cells.

Neurodegenerative disease—a disease characterized by a progressive decline in the structure, activity, and function of brain tissue. These diseases include AD, Parkinson’s disease, frontotemporal lobar degeneration, and dementia with Lewy bodies. They are usually more common in older people.

Neurofibrillary tangle—a filamentous collection of twisted and hyperphosphorylated tau found in the cell body of a neuron in AD.

Neuron—a nerve cell.

Neurotransmitter—a chemical messenger between neurons. These substances are released by the axon on one neuron and excite or inhibit activity in a neighboring neuron.

Nucleus—the structure within a cell that contains the chromosomes and controls many of its activities.

Oxidative damage—damage that can occur to cells when they are exposed to too many free radicals.

Positron emission tomography (PET)—an imaging technique using radioisotopes that allows researchers to observe and measure activity in different parts of the brain by monitoring blood flow and concentrations of substances such as oxygen and glucose, as well as other specific constituents of brain tissues.

Single photon emission computed tomography (SPECT)—an imaging technique that allows researchers to monitor blood flow to different parts of the brain.

Synapse—the tiny gap between nerve cells across which neurotransmitters pass.

Tau—a protein that helps to maintain the structure of microtubules in normal nerve cells. Abnormal tau is a principal component of the paired helical filaments in neurofibrillary tangles.

Thalamus—a small structure in the front of the cerebral hemispheres that serves as a way station that receives sensory information of all kinds and relays it to the cortex; it also receives information from the cortex.

Transgenic—an animal that has had a gene (like human APP) inserted into its chromosomes. Mice carrying the mutated human APP gene often develop plaques in their brains as they age.

Ventricle—a cavity within the brain that is filled with cerebrospinal fluid.

Vesicle—a small container for transporting neurotransmitters and other molecules from one part of the neuron to another.